There is well known a technology what is called a Media Key Block (MKB). A unique key ring (device key) is allocated to each of many devices. Common data called the MKB is distributed to each device. Each device processes the MKB using the device key allocated to the device. As a result of the MKB processing, each device acquires data called a media key. An arbitrary number of designated devices can be revoked using the MKB. For example, the MKB can be configured such that a device 8 and a device 21 are revoked. In this case, the media key is not acquired even if the MKB is processed using the device key retained by the device 8. The same holds true for the device 21.
The device key allocated to each device can individually be revoked using the technology of the MKB. The revoke can efficiently be made by a combination of the device keys to be revoked. Utilizing such a feature, the MKB is applied to a copyright protection technology. For example, a situation can be considered in which encrypted contents are illegally decrypted by illegally analyzing a sequence of devices having device keys and therefore the plain-text contents are leaked. Such illegal leak of the contents takes place in the case in which the media key is easily read from the outside because a sequence of devices produced by a certain manufacturer is short of robustness, for example.
If an owner of copyright of contents or an agent thereof learns the fact of the illegal leak, the owner of copyright of contents or the agent thereof distributes the MKB that revokes the sequence of devices having the device keys. Therefore, the sequence of devices are revoked and the media key cannot be derived in the revoked devices. In the case in which the media key derived from the MKB is utilized to decrypt the encrypted contents, the encrypted contents cannot be decrypted in the revoked device. The leak of contents from the sequence of devices having the problem with the robustness can be prevented by updating the MKB.